Shortcut to switch between add and subtract in weight paint mode?
You could change the brush: There are Add and Subtract brushes (and more for all blend modes) which will set all options of the selected brush including the blend mode. So you can even configure more settings of the brushes and select them with one shortcut.

The number keys (not on the number pad) are used as shortcuts for the brushes. You can select the Add brush with 1 and the Subtract brush with 9. They are numbered from one to nine in the same ordering as they appear in the brush selection.